A method has been developed for the quantitative determination of the rates of local glucose consumption in the discrete functional and structural components of the brain in conscious or anesthetized laboratory animals. The method is based on the use of (14C)deoxyglucose as a tracer for glucose flux through the hexokinase step. Local (14C)deoxyglucose-6-phosphate concentrations in the tissues of the CNS are measured by a quantitative autoradiographic method. All the theoretical and technical problems have been solved, and the method is now in use for the determination of normal values in the conscious and anesthetized rat. Inasmuch as the autoradiographs of the relative rates of local glucose consumption can be used directly for mapping functionally, and therefore metabolically, linked structures in the CNS, the method is being used to map the functional visual pathways in the monkey. It is also being used to study alterations in the energy metabolism of the discrete functional and structural components of the brain in a variety of physiological, pharmacological, and pathological states.